In an engine of a vehicle or the like, a piston ring is mounted on a piston and the piston ring is caused to slide over a wall surface of a sleeve (a cylinder) which is an opposite member. Accordingly, low friction and abrasion resistance are required for the piston ring, and a carbon film of diamond-like carbon (DLC) or the like has been formed on an outer circumferential sliding surface of the piston ring in the related art.
However, with recent increases in output power and rotation speed of engines, use of a piston formed of an aluminum (Al) alloy as a piston, and the like, there is a problem with abrasion of a piston ring groove. Therefore, in order to cope with this problem, techniques of forming a carbon film of diamond-like carbon or the like on top and bottom surfaces of a piston ring in addition to the outer circumferential sliding surface thereof have been studied (for example, see Patent Literatures 1 to 5).
At this time, formation of a carbon film on a piston ring is normally performed in a state in which a plurality of piston rings are arranged in a chamber of film formation equipment. However, the outer circumferential sliding surface and the top and bottom surfaces require different characteristics of the carbon films formed thereon. For example, low friction with a sleeve which is an opposite sliding member in addition to abrasion resistance is required for the outer circumferential sliding surface. On the other hand, low attackability and adhesion resistance for preventing burning due to scratching of a piston ring groove based on wobbling of the piston ring during operation in addition to abrasion resistance are required for the top and bottom surfaces.
Accordingly, in Patent Literatures 1 to 4, based on the premise that a film is formed on the outer circumferential sliding surface and the top and bottom surfaces of a piston ring under the same treatment conditions, the outer circumferential sliding surface and the top and bottom surfaces have the same film quality and have only a difference in a film-forming state depending on set positions at the time of film formation, and it cannot be said that satisfactory film formation corresponding to different characteristics required for the outer circumferential sliding surface and the top and bottom surfaces is performed.
Therefore, an example of a technique of forming carbon films with film qualities corresponding to different characteristics required for the outer circumferential sliding surface and the top and bottom surfaces of the piston ring has been proposed in Patent Literature 5. Specifically, in Patent Literature 5, when a hard carbon film is formed by plasma CVD, carbon films with different film qualities are simultaneously formed on the outer circumferential sliding surface and the top and bottom surfaces by setting piston rings in a chamber such that the top and bottom surfaces of the piston rings are parallel to an ion flow from a plasma source and the outer circumferential surfaces are perpendicular to the ion flow.
However, in this case, since the piston rings need to be arranged with an increased gap between the top and bottom surfaces of the piston rings such that film-forming states of the carbon films which are formed on the surfaces are not affected, the number of piston rings which can be arranged in one chamber may decrease and cause a decrease in productivity. Even when a plurality of piston rings are arranged in this way, it is not easy to optimize different film qualities for the outer circumferential sliding surface and the top and bottom surfaces.